H.264, also referred to as MPEG-4 (Motion Picture Expert Group) AVC (Advanced Video Coding), is the state of the art video coding standard. It is a hybrid codec that is based on eliminating redundancy between frames, denoted so-called inter coding, and within frames, denoted so-called intra coding. The output of the coding process is VCL (Video Coding Layer) data, which is further encapsulated into NAL (Network Abstraction Layer) units prior transmission or storage.
In H.264/MPEG-4 AVC a picture of a video stream is composed of macroblocks with a fixed size of 16×16 pixels and the coding of the picture proceeds macroblock by macroblock. Each picture of the video stream is divided into one or more slices. A slice is an independently decodable piece of the picture. This means that if one slice of a picture is lost, the other slices of the picture are still decodable. In addition, slices can be used for parallel coding and decoding since they are independent from other slices in the picture. In H.264/MPEG-4 AVC a slice boundary must occur between the border of two adjacent, according to the coding order, macroblocks.
HEVC (High Efficiency Video Coding) is a successor to H.264/MPEG-4 AVC. HEVC aims to substantially improve coding efficiency compared to H.264/MPEG-4 AVC, i.e. reduce the bitrate requirements while keeping the picture quality. HEVC is targeted at next-generation HDTV (High Definition Television) displays and content capture systems which feature progressive scanned frame rates and display resolutions from QVGA (Quarter Video Graphics Array) (320×240) up to 1080p and Ultra HDTV (7680×4320), as well as improved picture quality.
HEVC enables usage of so-called largest coding units (LCUs) that are blocks of pixels having a size that is larger than the macroblocks of H.264/MPEG-4 AVC to provide improved coding efficiency. In order to handle both large homogenous areas and small detailed areas in the same picture a hierarchical coding has been proposed for HEVC. The LCUs in a picture are scanned in a predefined order and each such LCU may be split into smaller coding units (CUs), which in turn may be split hierarchically in a quadtree fashion down to a smallest coding unit (SCU). A picture may, thus, be encoded as a mixture of coding units with different sizes ranging from the LCU size down to the SCU size.
In correspondence to H.264/MPEG-4 AVC a picture of a video stream can be divided into one or more slices in HEVC. The slice boundary is in HEVC aligned with the border of two adjacent, according to a predefined order, LCUs. However, since the LCUs can be at least 16 times larger than the macroblocks in H.264/MPEG-4 AVC the resolution or granularity at where slice borders can be set have actually decreased in HEVC. This can result in slices that are far from being optimally sized for transport mechanisms, such as either too small or too large. Unnecessarily small slices increase the overhead since each slice comes with a certain overhead cost. A too large slice implies that the slice has to be split on the transport level, which results in decreased error resilience since then parts of a slice can be lost in the transmission with the consequence that the slice is not decodable.
There is therefore a need for an efficient management of slices in HEVC and in pictures with large sizes of pixel blocks.